1. Field of the Invention
This invention pertains to devices and methods for producing electric glow discharges, preferably for discharge pumping of gas lasers. More particularly, this invention pertains to the use of charged transmission lines or capacitors as energy sources for the discharge.
2. Description of the Prior Art
A charged transmission line is conventionally used as an energy source for discharge-pumped gas lasers. Such lasers generally include a gaseous laser gain medium within which is positioned a pair of discharge electrodes. When the charged transmission line is connected to the discharge electrodes, the voltage on the transmission line initiates a glow discharge in the gas between the electrodes, whereby a portion of the energy stored in the transmission line is transferred to the gas. Under appropriate circumstances, the energized gas will exhibit lasing action.
In most discharge-pumped lasers, the initial "firing" voltage across the discharge electrodes that is required to initiate the glow discharge is significantly greater than the "sustaining" voltage required to sustain the glow discharge thereafter, and it is commonly as much as six times greater. If, as in the prior art, the transmission line is charged to a high enough voltage to initiate or "fire" the glow discharge, the transfer of energy from the transmission line to the glow discharge will be relatively inefficient. The reason for this inefficiency will become clear from the following operational description of the prior art.
In operation, once the charged transmission line is connected to the discharge electrodes, the glow discharge initiates or "fires", and the voltage across the discharge electrodes rapidly drops to a lower steady-state or "sustaining" voltage. This rapid voltage drop causes a traveling wave to propagate along the transmission line from the end of the line connected to the discharge electrodes to the opposite, open end of the transmission line. At that point, the traveling wave is reflected and propagates back toward the discharge electrode end of the line.
The traveling wave is approximately a voltage step function whose amplitude equals the initial voltage of the charged transmission line minus the steady-state voltage of the glow discharge, and whose polarity is opposite that of the initial voltage of the transmission line.
If the voltage of the wave were approximately one-half of the initial voltage to which the transmission line was charged, the line would be completely discharged by the round trip propagation of the wave from the discharge end to the open end and return. However, in the prior art, in order to initiate the glow discharge, the transmission line is charged to an initial voltage much greater than twice the steady state voltage of the glow discharge. Consequently, in the prior art the traveling wave initially has a magnitude of much greater than one-half the initial voltage of the transmission line. The round-trip propagation of this wave from the discharge end of the transmission line to the open end and back leaves a partially charged transmission line (its polarity being reversed from the initial charge).
If the voltage remaining on the transmission line is sufficient to sustain the discharge, the transmission line may continue to transfer additional energy to the discharge. However, the energy will be transferred over an extended period of time, which results in inefficient generation of lasing action. Also, if the energy is transferred over an extended period of time, the more or less uniform glow discharge tends to break down into arcs, which do not properly excite the laser medium. When the voltage remaining on the transmission line is insufficient to sustain the glow discharge, the energy represented by the remaining voltage is left, untransferred, on the transmission line. The end result, in the prior art, is an inefficient transfer of energy from the transmission line to the glow discharge.